The invention relates to a base station for a radio system, the base station comprising, for monitoring the quality of a radio link between the base station and mobile stations, generator means for generating a square wave signal having a desired monitoring signal frequency; control means for setting the frequency of the square wave signal; a filter means for generating a monitoring signal from the square wave signal; and a transmitter means for transmitting the monitoring signal to the mobile radio station.
In the Nordic Mobile Telephone (NMT) system, the quality of the radio link between a base station and a mobile telephone is monitored by means of special monitoring signals. The base station generates a monitoring signal by lowpass filtering from a clock signal having a desired monitoring signal frequency. The NMT system utilizes four monitoring signal frequencies at intervals of 30 Hz. The base station transmits a monitoring signal to a mobile telephone which returns the monitoring signal immediately to the base station, and the base station determines the quality of the used radio link, especially the signal-noise ratio, by means of the received monitoring signal. A radio system base station utilizing such a monitoring signal is disclosed in FI Patent Application 900620.
The new NMT systems include recommendations (ref. Addendum to NMT-Specification DOC 900-4 DRAFT dated 8.1. 1991) stating that a digital monitoring signal should be used in addition to an analog monitoring signal, and that the bit error ratio of the received digital monitoring signal should be used in addition to its S/N ratio as a criterion of quality. When the quality of the speech connection is above a predetermined threshold value, a digital monitoring signal is used, and when it is below the threshold value, an analog monitoring signal is used. The digital monitoring signal should be modulated so that the frequency 3,975 Hz corresponds to the digital logical state "1", and the frequency 4,025 Hz corresponds to the logical state "0". The decision to handover the call from one base station to another is also made on the basis of the monitoring signal.